The present invention relates to the filtration of molten metal.
Molten metal, particularly molten aluminum, in practice generally contains entrained and dissolved impurities, both gaseous and solid, which are deleterious to the final cast product. The impurities may originate from several sources. For example, the impurities may include metallic impurities such as alkaline and alkaline earth metals, and occluded hydrogen gas and dissolved surface oxide films which have become broken up and are entrained in the molten metal. In addition, the inclusions may originate as insoluble impurities, such as carbides, borides and others or eroded furnace and trough refractories.
In typical sand, permanent mold and die casting foundries, it is common practice to use a pot or crucible type melting and holding furnace in capacity from 300 lbs to 1500 lbs of molten aluminum alloy. Depending on the type of operation, this furnace may be filled with the molten alloy from a larger furnace, or, the cold alloy may be added to the crucible furnace and melted therein. After the furnace is full of molten metal, it is common practice to treat the melt by fluxing with Cl.sub.2, N.sub.2, mixtures thereof, or C.sub.2 Cl.sub.6 and an added additional grain refiner in the form of salts or a 5% Ti-1% B aluminum alloy hardener. The melt is then adjusted to the desired temperature and pouring of castings is allowed to begin.
Pouring is usually carried out with hand carried or manipulated ladles. The ladle is sized to hold slightly more metal than that required to pour one or more of the molds. The operator dips the ladle into the melt and fills it, wipes any skim from the melt surface and ladle lip and pours the molten metal into the molds. The residue of metal left in the ladle is dumped back into the parent melt in the furnace. Numerous pours are made in this manner until 1/2 to 2/3 of the molten metal has been cast. The repeated operation of bailing, pouring and dumping back results in the generation and entrainment of large quantities of oxide films and particulate in the parent melt.
The foundry alloy ingot used in preparing melts as described above are normally cast by a primary or secondary producer in an inline pigging machine. This operation involves the free fall of metal into the pig mold cavity. In addition, the melt treatment practices generally used in the production of the foundry alloy ingot are less than adequate. As a result, the ingot usually contains entrained oxide films and non-metallic particulate.
The oxides generated and entrained in the melt as described above can and often do produce defects in the resulting castings that are a cause for rejection. This is particularly true in specification type work for critical applications where the foundryman must meet specified radiographic standards.
The use of proper melt treatment and fluxing practices by the foundryman helps minimize inclusion problems. However, there is no method presently available for insuring that metal ladled from the crucible type furnace and poured into the mold is substantially free of undesirable non-metallics.
It is naturally highly desirable to filter the molten metal in the crucible type furnace in order to remove or minimize impurities in the final cast product especially, for example, when the resultant metal is to be used in a decorative product, such as decorative trim, or products bearing critical specifications, such as aircraft forgings and extrusions, and light gauge foil stock. Impurities as aforesaid cause loss of properties such as tensile strength and corrosion resistance in the final solidified alloy and lead to degradation of processing efficiency and loss of properties in the final cast product.
One conventional method of filtering in crucible type melting operations is disclosed in Volumn 3 of "Aluminum" published by American Society of Metals, 1967, page 35. This procedure requires the placing of a filter plate of porous refractories or carbon vertically in a crucible thereby dividing the crucible into two compartments, one for charging with molten metal and the other for ladling the molten metal. This procedure has a number of drawbacks, among them, the filter of porous refractories or carbon is not efficient in removing the aforesaid oxide films and non-metallic particulate. Furthermore, the system is not particularly efficient because due to the limited surface area of the filters employed, the system must be continually shut down in order to replace the filters. Finally, there is a problem in sealing the filter in the crucible so as to prevent leakage of the molten metal around the filtering system.
An alternative to the above crucible filtering techniques as set out above is diclosed in U.S. Pat. No. 3,729,097. This procedure requires placing a free floating member with an aperture at the bottom, which is covered with a glass cloth filter, in a molten metal thereby allowing the metal to float up through the filter into an inclosure where it is removed by a ladle. Again, this procedure suffers from a number of deficiencies, for example, the limited size of the filter requires that it be replaced frequently while the filter material itself is not efficient in removing oxide films and non-particulate impurities.
Porous ceramic foam materials are known to be particularly useful in filtering molten metal, as described in U.S. Pat. No. 3,893,917 for "Molten Metal Filter" by Michael J. Pryor and Thomas J. Gray, patented July 8, 1975 and also as described in U.S. Pat. No. 3,962,081 for "Ceramic Foam Filter" by John C. Yarwood, James E. Dore and Robert K. Preuss, both of which patents are assigned to the assignee of the present invention.
Porous ceramic foam materials are particularly useful for filtering molten metal for a variety of reasons included among which are their excellent filtration efficiency, low cost, ease of use and the ability to use same on a disposable, throwaway basis. The fact that these ceramic foam filters are convenient and inexpensive to prepare and may be used on a throwaway basis allows for the development of means for easily assembling and removing porous molten metal filters from a crucible type melting furnace while providing a highly efficient filtration assembly.
Accordingly, it is the principal object of the present invention to provide an improved method and apparatus for the filtration of molten metal in a crucible type melting furnace.
It is a particular object of the present invention to provide an improved removable filter means for use in the filtration of molten metal in a crucible type furnace.
It is still a further object of the present invention to provide improvements as aforesaid which are convenient and inexpensive to utilize and which result in high filtration efficiency.
Further objects and advantages of the present invention will appear hereinbelow.